The Office for the Community & Voluntary Sector has a comprehensive list of New Zealand research into volunteering. Includes a "volunteering research stocktake".
The American Sociological Association also has volunteering related research on their site but it takes a patient and competent researcher to locate it (and some articles are only accessible by members).
One interesting article on the above site is "The motivations to Volunteer: Theoretical and Practical Considerations" (1999). It includes a list of further reading on the topic.